A wide variety of optical information recording and/or reproducing apparatus optically recording and/or reproducing the information, using an optical information recording medium, such as an optical disc, are currently in use. In particular, the recording and/or reproducing apparatus for an optical recording medium, employing the optical disc as the recording medium, is used extensively and researches into a higher recording density of the optical disc are now underway.
For example, as a replay-only optical recording and/or reproducing apparatus, such an optical disc apparatus, is now in use, which, with the use of a DVD (digital versatile disc) as a disc, allows for replay having a recording capacity as high as 4.7 GB equal to approximately seven times that of a CD-ROM, with the recording capacity of approximately 650 MB, even though the DVD is of the same diameter of 120 mm as that of a CD-ROM.
In general, an optical disc has a recording surface on a transparent substrate, and permits recording and/or reproduction of information signals by a light beam for recording and/or reproduction being illuminated through an objective lens on an optical disc and being transmitted through the transparent substrate so as to be converged on the recording surface of the disc.
On the recording surface of the optical disc, there are formed track-guiding grooves and lands for a light beam to correctly scan the recording track, these grooves or lands being used to detect tracking errors. A pit string, formed on the recording track, can also be handled as intermitted grooves or lands.
In addition to the aforementioned optical discs, the DVD-RAM, which can be rewritten freely, is in use, so that a demand is raised for a reproducing head for a DVD enabling this DVD-RAM to be reproduced and for a recording and/or reproducing optical head for a DVD-RAM enabling the DVD and the compact disc (CD) to be reproduced.
Meanwhile, in a conventional replay-only DVD or CD, or in a magneto-optical disc, the recording system used is such a one in which the information is recorded in only one of the land and the groove. On the other hand, with the DVD-RAM, the land-groove recording system of recording the information in both the land and the groove is used, with a view to elevating the recording density. In addition, a variety of recording mediums, employing the land and groove recording system, nave been proposed as a system for realization of the high density recording. The optical recording medium of this land-groove recording system affords certain broader widths to both the land and the groove, in distinction from the conventional magneto-optical disc in which the one of the land and the groove used for recording is of a broader width, with the other of the land and the groove being narrower in width.
However, in a optical recording medium of recording information signals by the land and groove recording system, it has been ascertained that the phenomenon termed “tracking interference” as later explained occurs in case of employing the aforementioned astigmatic method for detecting the focussing errors, thus giving rise to a noise termed “track traversing noise”.
This “tracking interference” is a phenomenon in which focussing error signals undergo significant changes in the focussing error signals when a beam spot traverses a track. The “track traversing noise” is a noise produced by the focussing error signals assuming different values depending on whether the beam spot is on the land or on the groove of the recording medium.
Referring to FIG. 1, the aforementioned “tracking interference” is elucidated by referring to FIG. 1.
Referring to FIG. 1, the abscissa and the ordinate denote object lens positions in a direction perpendicular to the disc, and an output level of the focussing error signals. A curve FEL, indicated buy a solid line, is a focussing error curve showing the relation between the objective lens position and the focussing error signals FE with the beam spot lying on the land of the optical disc, whilst a curve FEG, indicated by a broken line, is a focussing error curve showing the relation between the objective lens position and the focussing error signals FE with the beam spot lying on the groove of the optical disc.
Referring further to FIG. 1, a peak-to-peak range of the focussing error curve FEL (FEG) is defined as a focussing pull-in range Spp. It is within this range only that focussing servo occurs. The reason this focussing pull-in range Spp is provided and focussing servo is made to occur only in this range is that, since the focussing error signals may be zero even when the objective lens position is shifted appreciably from the focussing position, it is necessary to evade the defocussed state being detected as being the focussed state.
Still referring to FIG. 1, the focussing error signals FE in the focussing pull-in range Spp assume different values, depending on whether the beam spot is on the land or on the groove of the optical disc. So, there are two positions of the zero focussing error signals FE, namely a position corresponding to the objective lens position XL when the beam spot is on the land and a position corresponding to the objective lens position XG when the beam spot is on the groove.
On the other hand, a controller for controlling the optical head operation controls the current supplied to a lens driving coil, and drives the objective lens along its optical axis so that the focussing error signals FE will be zero. So, each time a beam spot is moved from the land to the groove on the optical disc, or vice versa, the objective lens is reciprocated between the positions XL and XG, thus producing the track traversing noise. This noise is responsible for a variety of inconveniences, such as defocussing, worsened transmission characteristics in the focussing or tracking servo, or scorching or destruction of the lens driving coil.
Meanwhile, the mechanism responsible for the tracking interference phenomenon, as explained with reference to FIG. 1, has not been analyzed sufficiently to date.
For alleviating the inconvenience, brought about by the tracking interference phenomenon, it may be contemplated to detect focussing errors using the so-called spot size method.
That is, the aforementioned astigmatic method produces a signal corresponding to the shape of a received light spot by summation and subtraction of output signals from respective photodetector segments, whilst the spot size method detects the spot size by the output signals of light receiving sections to perform focussing servo control based on the spot size.
There is also a differential push-pull method as a system for producing tracking error signals in stability. In this case, however, three light spots are used as beam spots converged on the recording medium.
If the land-groove recording system is used, such a structure is desirable which is based on detection of a land-groove decision signal used for verifying on which of a track on a land and a track in a groove the light beam is converged. That is, it may be extremely difficult to realize a simplified structure which is based on detection of the focussing error signals or the land-groove decision signal in the differential push-pull method and on detection of the focussing error signals by the spot size method.